Ventilators for patients requiring breathing assistance have traditionally been large, heavy, power-hungry devices that have provided little, if any, mobility to a patient. Recent advances in compressor technology, such as those described in U.S. Pat. No. 6,152,135 issued to DeVries et al., have allowed a reduction in size and power requirements of ventilators, for the first time allowing the manufacture of ventilators that were able to provide a limited degree of self-contained portability. Outfitted with battery packs, these portable ventilators could be attached to a wheel chair, providing a patient the ability to move about for a limited amount of time without having the ventilator connected to a power supply main.
Ventilators of the prior art have become smaller and more transportable while maintaining the ability to provide complex gas delivery modes to a patient, typically by using low pressure rotary drag compressors as the gas delivery mechanism. Such drag compressors may either be variable speed or constant speed. Variable speed ventilator compressors operate by rapidly accelerating from a standstill to provide inhalation assistance (inspiration) to a patient, then decelerating rapidly to allow a patient to exhale. The rapid acceleration and deceleration of prior art variable speed compressor ventilators require the compressor's drive circuitry to handle very high currents, necessitating bulky and expensive power systems, and consuming considerable battery power when the ventilator is not connected to a power main.
Constant speed compressors do not need the bulky power systems of variable-speed compressors, but have inherent inefficiencies because the compressor continues to run and consume power even at times when no gas is being supplied to the patient (such as during exhalation). The power consumption can be reduced by recirculating the compressor's output gas flow to the compressor's intake during exhalation. However, even the reduced power consumed significantly reduces the amount of time the ventilator can be operated from on-board battery power.